Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic process.
Description of the Related Art
Image forming apparatuses using an electrophotographic or electrostatic recording method have used a corona charger as a unit for charging an image-bearing member such as an electrophotographic photosensitive member or an electrostatic recording dielectric.
In recent years, a contact charger has been put into practice as the charge processing unit of an image-bearing member because of its advantages of low ozone production and low power consumption. The contact charger charges a charge target member in such a manner that a charging member to which a voltage is applied is brought into contact with the image-bearing member.
The charge method used for such a contact charger includes a “direct-current (DC) charge method” by which only a DC voltage is applied to a charging member to charge a charge target member. The charge method also includes an “alternating-current (AC) charge method” by which a charge target member is charged by applying an oscillation voltage that has an AC voltage component and a DC voltage component and whose voltage value periodically changes with time. In recent years, the “AC charge method” having good charge uniformity has been used widely.
When an image forming apparatus using such an AC charge method performs charge control, the image forming apparatus alternately applies positive and negative voltages and repeats discharge and back discharge. Accordingly, the discharge increases deterioration of a photoconductor drum that is a charged member on its surface. The deteriorated surface portion of the photoconductor drum is shaved due to friction with an abutting member such as a cleaning blade, and the life of the photo conductor is thus decreased.
Hence, many methods for controlling and minimizing a discharge current amount in the AC charge method have been proposed (for example, Japanese Patent Laid-Open No. 2010-231188).
An image forming apparatus proposed in Japanese Patent Laid-Open No. 2010-231188 uses a high-pass filter to extract a discharge current component from a current that flows between the photo conductor and the charger when an AC voltage is applied to the charger. Based on the extracted discharge current component, a peak-to-peak voltage value of the AC voltage is controlled.